Diabetes is a disease of carbohydrate metabolism characterized by hyperglycemia and glycosuria, resulting from inadequate peripheral action of insulin. The most significant long-term pathological consequence of diabetes, generalized basement membrane thickening, requires many years to develop. Because of this long interval between the derangement of carbohydrate metabolism and the morphologic changes in basement membrane, the exact causal relationship between them remains controversial. In order to clarify this relationship, we propose to utilize a model system in diabetic rats, the parietal yolk sac, characterized by rapid basement membrane turnover, this system will facilitate the study of basement membrane changes in diabetes and allow for a better definition of the relationship between insulin treatment and microangiopathy.